Exhaust gas recirculation (EGR) is a method of reducing emissions of oxides of nitrogen (NOx) from an engine by returning a portion of an engine's exhaust gas the engine combustion chambers via the air intake. EGR works by lowering the oxygen concentration in the combustion chamber, thereby decreasing the peak temperature of the fuel combustion flame as well as through heat absorption. EGR is not a new technology—it has been used since the mid-1970s in gasoline fueled passenger car engines. Following the gasoline application, EGR was also introduced to diesel passenger cars and—from the early 2000s—to heavy-duty diesel engines.
Generally, there are two exhaust system arrangements comprising EGR: (i) high pressure loop EGR, in which the exhaust gas is recirculated from upstream of a turbocharger to ensure that exhaust gas will flow from the former to the latter; and (ii) low pressure loop EGR (also called long loop EGR), where exhaust gas is often recirculated from downstream of a particulate filter, allowing all the exhaust gas to be utilised in the turbo. Exhaust gas pressure downstream of the filter is generally lower than at the intake manifold, allowing exhaust gas to flow from the former to the latter location.
In use, particularly during cold start in a vehicle configured to meet the MVEG-A drive cycle, an EGR valve is set to recirculate approximately 50% of the exhaust gas to the engine. Exhaust gas emitted from the engine during EGR has a lower oxygen content but a no higher NOx content than exhaust gas recirculated from the exhaust system to the engine.
WO 2008/047170 discloses a method of reducing oxides of nitrogen (NOx) present in a lean gas stream comprising nitric oxide (NO), the method comprising the steps of: (i) net adsorbing NO per se from the lean gas stream in an adsorbent comprising palladium and a cerium oxide at below 200° C.; (ii) thermally net desorbing NO from the NO adsorbent in a lean gas stream at 200° C. and above; and (iii) catalytically reducing NOx on a catalyst other than the NO adsorbent with a reductant selected from the group consisting of a hydrocarbon reductant, a nitrogenous reductant, hydrogen and a mixture of any two or more thereof. There is also disclosed a system for reducing NOx in a lean gas stream comprising NO, which system comprising an adsorbent for adsorbing NO per se from the lean gas stream at below 200° C., means for contacting the NO adsorbent with a lean gas stream at 200° C. and above thereby to desorb NO from the NO adsorbent and means for reducing NO desorbed from the NO adsorbent comprising a NO reduction catalyst and a source of reductant selected from the group consisting of a hydrocarbon reductant, a nitrogenous reductant, hydrogen and mixtures of any two or more thereof, wherein the NO adsorbent comprises palladium and a cerium oxide.
We now propose an exhaust system arrangement that can improve NOx conversion over a legislative drive cycle (such as the European MVEG-A drive cycle) and in real world conditions which can lower the NOx emissions from vehicular lean burn internal combustion engines relative to current commercial exhaust system arrangements.